1. Field of the Invention
The present disclosure relates to bus bar assemblies.
2. Description of the Related Art
Bus bars are structures that electrically couple electrical elements, such as capacitors and solid state devices, in an electric circuit.
As is well known in the art, any electric current may have an associated magnetic field. The magnitude of the associated magnetic field is proportional to the magnitude of the current. In a uniform material, the magnitude of the magnetic field is also proportional to the inverse of the square of the distance from the current. The direction of the magnetic field is perpendicular to the direction of current.
Where the current changes as a function of time, the associated magnetic field may also change as a function of time. A changing magnetic field may in turn produce a changing magnetic flux. A changing magnetic flux may induce an emf in a circuit. The magnitude of the induced electromagnetic field (emf) is directly proportional to the time rate of change of the magnetic flux as given by the following equation:
  emf  =      N    ⁢                  ⅆ                  Φ          m                            ⅆ        t            where N is the number of turns of wire in an inductor (which may be equal to 1 for a bus bar), Φm is the magnetic flux and t is time.
An induced emf may be undesirable in an electric circuit as it may damage electrical circuit elements. Therefore, as given by the above equation, to reduce undesirable induced emfs, one may seek to reduce the changing magnetic flux.
One method of reducing the changing magnetic flux at a particular point is to geometrically arrange current carrying elements so that their associated magnetic fields are of equal magnitude and aligned in opposite directions. One way to do this is to arrange current carrying elements in a substantially parallel, proximate orientation where their currents run in opposite orientations. This is a technique known in the art.
Industrial capacitors are often physically large as compared to other electric circuit elements. Simply due to their size, shape and physical design, it has heretofore been difficult to arrange multiple capacitors so as to reduce the magnetic flux in the manner described above. Capacitors are of particular concern because a discharging capacitor may produce a very high current that varies at an extremely rapid rate inducing a very large emf that may damage other elements in the circuit. For example, the current in a discharging commercially available capacitor may change from 140 A to 28 A in 100 nanoseconds yielding an extremely high rate of change of 1,120,000 A per second. FIG. 1 shows a graph of current versus time for a discharging commercially available capacitor.
Apparatus in the art have attempted to arrange multiple capacitors with bus bar structures so as to reduce magnetic flux and induced emfs however, such apparatus do not reduce the net magnetic flux to a sufficient degree. Japanese Patent Application No.: 2004043398, for example, aligns single pairs of bus bar mounting arms proximate and parallel to one another. Japanese Patent Application No.: 08142168, as another example, aligns multiple pairs of bus bar mounting arms but offsets the mounting arms along their length such that they are not proximate over their lengths.
Others have addressed the problem of induced emf in another fashion and have not attempted to reduce magnetic flux but instead, have opted to design robust electric circuits so as to avoid damage. Apparatus known in the art that are robust include insulated gate bipolar transistors (IGBT). IGBTs are rated for such higher voltages and have tolerance of such voltage overloads and have been used, for example, in hybrid gasoline electric automobiles. IGBTs unfortunately, are expensive and have high losses.
Therefore, there remains a need in the art for a compact, inexpensive bus bar structure that has the effect of reducing an associated magnetic flux. The embodiments disclosed herein address these needs and provide associated benefits.